Synthesis, spectroscopy, and crystallography of magnesium, aluminum, and zinc complexes supported by a tridentate ketoiminate

Abstract

The Schiff base condensation of 8-aminoquinoline with 2-acetylcyclopentanone yielded two regioisomeric β-ketoimines with a cyclopentanyl ring in the ketoimine backbone and a pendant quinolyl moiety. The major ketoimine, isolated in 64% yield, showed quinolyl addition to the cyclopentanyl ring, and it was characterized spectroscopically and crystallographically. This ketoimine was utilized to prepare a series of bis-ligated homoleptic magnesium, aluminum, and zinc complexes in 78–96% yield as well as aluminum and zinc ketoiminate phenoxide complexes in 68 and 66% yield, respectively. The five complexes were characterized spectroscopically and crystallographically, and the bis-ligated complexes showed distorted octahedral geometries. The aluminum phenoxide complex showed a monomeric, five-coordinate metal center (τ5 = 0.83) while the zinc phenoxide complex displayed a dimeric structure with two five-coordinate zinc centers (τ5 = 0.39 and 0.53) with bridging phenoxide and ketoiminate ligands, where idealized trigonal bipyramidal geometry τ5 = 0.00 and idealized square pyramidal τ5 = 1.00. The zinc phenoxide showed unexpected fluxional bonding character which was explored with VT-NMR from 220 to 325 K and is proposed to be dimer disproportionation to monomeric zinc complexes. Density Functional Theory (DFT) and Time-Dependent DFT calculations were used to investigate the regioisomeric ratio of the ketoimines and map the electronic structures for the ketoimines and complexes, providing insight into the nature of the observed electronic transitions.